Walls made from gypsum wallboard are conventionally constructed by attaching the wallboard panels to studs, and filling and coating the joints between the panels with a specially formulated composition called a joint compound. The wetted joint compound (drying type; taping grade) is placed within the joint formed by the abutting edges of the wallboard panels, and a paper reinforcing tape is embedded in the joint with the joint compound which is then permitted to dry. When the joint compound is dry, a second joint compound (topping or finishing grade) is applied over the joint, and it too is permitted to dry. It is usually necessary for a third application of a joint compound and, after it is dry, it may be lightly sanded and may be conventionally finished with a decorative material (paint, texture or wallpaper) then applied to the wall. Instead of using two grades of joint compound, an all purpose joint compound is commercially available which is used both for embedding the tape and for the finish coat(s). If a fiberglass reinforcing tape is used, it is applied to the wall prior to the application of joint compound and is used with chemically setting type joint compound. For the taping step the setting type joint compound is applied over the fiberglass tape and forced through the tape.
Conventionally, all joint compounds contain a filler, a binder and a thickener. The taping grade joint compound generally contains more binder than the topping grade. The conventional fillers are calcium carbonate, calcium sulfate dihydrate (gypsum), and calcium sulfate hemihydrate (plaster of Paris). The calcium sulfate hemihydrate is used only in setting type joint compounds as disclosed in U.S. Pat. No. 3,297,601. In current construction practices, generally it is preferred to use a ready-mixed, pre-wetted, drying type joint compound which contains either a calcium carbonate or gypsum filler. The ready-mixed, pre-wetted, drying type joint compounds are preferred for the convenience of avoiding the problems of mixing powdered materials into water at a jobsite: airborne dust, messy floors and workspace, lump removal to achieve a smooth mixture, and extra time. However, these conventional drying-type joint compounds encounter shrinkage during drying which make it more difficult to achieve a smooth wall surface, and each coat of drying type joint compound must dry thoroughly before additional coats can be applied.
In order to overcome the disadvantages of the drying-type joint compounds, setting-type joint compounds possess inherently low shrinkage and can be recoated after setting allowing for faster construction. Powdered setting-type joint compounds have the disadvantage of needing to be drill mixed with water on the jobsite at the time of use. Jobsite drill mixing of powdered compounds generates excess dust and mess, eliminating lumps to achieve a smooth mixture can be difficult and more time is needed compared to using a ready-mixed compound. The prior art in U.S. Pat. No. 5,746,822, herein incorporated by reference, discloses a ready-mixed, setting type joint compound. Set preventers are useful for interrupting the chemical conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate. The ability to interrupt the chemical setting process enables the production of a setting-type joint compound that is in a ready-mixed form instead of a powder. Setting reactions that had been interrupted in the ready mixed setting compound through the use of a set preventer can be made to resume upon addition of a set activator. The advantages of a preferred ready-mixed, pre-wetted product are thus incorporated into a product with the shrinkage and recoating advantages of a setting type joint compound. U.S. Pat. No. 5,746,822, describes a two component composition with a premixed, cementitious component comprising a wetted calcium sulfate hemihydrate, conventional joint compound binders and thickeners, and a set preventer for a cementitious base. The other component, a set activator such as zinc sulfate, restarts the chemical setting reactions previously interrupted by the set preventer. Zinc sulfate is a slow-acting set activator. Even when used in relatively large amounts, joint compounds activated with zinc sulfate take long times to chemically harden in strength. Applicators must wait for the joint compound coat to be sufficiently firm and hard for application of a subsequent coat, sometimes until the following day.
Alum is known as a chemical set accelerator for calcium sulfate hemihydrate systems. It hastens setting hydration reactions in mixtures of calcined gypsum and water. However, alum is rarely used because excessive foam is generated in the presence of calcium carbonate. Calcium carbonate can be present either as a naturally occurring contaminant of other raw materials or as an added component. In joint compounds, this type of foam generation causes loss of strength and surface cratering. In addition, dramatic volumetric expansion from generation of the foam can cause the joint compound to expand up and out of the mixing pail, and if the expansion continues after the compound has been applied to the wallboard surface, the finished joint can expand to create unsightly convex jointwork (crowned joints). The teachings of the prior art teach away from using alum in stuccos containing calcium carbonate, but refrain from suggesting any other solution to the problem.